This invention relates to aqueous alkaline zincate solutions and to a process for depositing a zincate coating on aluminum or aluminum alloy substrates. The invention also relates to metal plated aluminum or aluminum alloy substrates.
One of the fastest growing worldwide markets is the processing and plating of aluminum and its alloys. Aluminum""s unique physical and mechanical characteristics make it particularly attractive for industries such as automotive, electronics, telecommunications, avionics, along with a plethora of decorative applications. Among aluminum""s most endearing properties include it""s low overall density (2.7 g/cc), high mechanical strength achieved through alloying and heat treating, and its relatively high corrosion resistance. Additional properties include; high thermal and electrical conductance, its magnetic neutrality, high scrap value, and its amphoteric chemical nature. Most aluminum components are made from aluminum alloys with alloying elements including: silicon, magnesium, copper, etc. These alloying mixes are formed in order to achieve enhanced properties such as high-strength or ductility.
The plating of aluminum and its alloys require specific surface preparations for successful electrolytic and electroless deposition. The most common practice used in order to achieve successful electrodeposition is applying an immersion zinc coating (better known as zincate) to the substrate just prior to plating. This procedure has long been considered the most economical and practical method of pre-treating aluminum. The major benefits of applying a zincate layer for pretreatment are the relative low cost of equipment and chemistry, wider operating windows for processing, and ease of applying a controlled deposit.
The presence of other metals in the zincate solutions has an affect on the rate and efficacy of the zinc deposition. Small amounts of alloy components (i.e. Fe, Ni, Cu) improve not only the adhesion of the zincate deposit, but also increase the usability of the zincate on a variety of aluminum alloys. Hence, the addition of Fe ions improves the adhesion on magnesium containing alloys. The presence of nickel in the zincate improves the adhesion of nickel plated directly onto the zincate, and similar effects can be found with addition of copper in the zincate and subsequent copper plate. In general, however, the alloying of zincate has shown to provide thinner and more compact deposits which effectively translate into better adhesion of downstream electroless/electrolytic plating. On the other hand, the composition of an alloying zincate becomes more and more complicated with the additional metal ions in the composition. It makes selection of complexing agents more complicated and critical for the overall performance of the zincate. Zinc-iron-nickel compositions are more sensitive than zinc-iron compositions for the selection of complexing agents and ratio of metal ions in the composition. This becomes even more critical with the addition of the cooper ions in the alloy zincate. Due to its noble position in the galvanic series, the deposition rate of copper in the immersion zincate deposition is much higher than the other elements in the zincate. Therefore, control of the deposition rate of copper becomes important. It is possible to control the deposition rate of copper by the selection of the right complexing agent(s) for copper ions and adequate ratio with the other metal ions. There are few strong complexing agents for copper ions which offer good stability and performance of the alloying zincate, and cyanide appears to be the best candidate. Cyanide is a complexer of choice for the copper containing zincate compositions and it has been the industry standard for that application for many years. A negative aspect for the use of cyanide is the extremely toxic nature of cyanide, and therefore, like other metal finishing products, the search for a cyanide replacement in the alloying zincate has been a topic of interest for many years.
The present invention provides an improved aqueous alkaline zincate solution comprising hydroxide ions, zinc ions, nickel ions and/or cobalt ions, iron ions, copper ions, and at least one inhibitor containing one or more nitrogen atoms, sulfur atoms, or both nitrogen and sulfur atoms provided said nitrogen atoms are not present in an aliphatic amine or hydroxylamine. The present invention also relates to methods for depositing zincate coatings on aluminum and aluminum alloys comprising applying an immersion zincate coating on an aluminum or aluminum alloy substrate, optionally followed by plating the zincate coated aluminum or aluminum alloy substrate using an electroless or electrolytic metal plating solution.